This invention relates to a communication system.
In some known communication networks including a plurality of units or stations connected via a common transmission line, when a first station requires a communication with a second station, the first station transmits a locking (holding) packet to the second station to lock (hold) the second station into a state where the second station accepts a subsequent packet or packets from the first station but rejects any packets from other stations. Upon the receipt of the locking packet, the second station recognizes that the second station is locked by the first station. At an end of the communication, the first station transmits an unlocking (unholding) packet to the second station to release the second station from the locked state. When the second station moves out of the locked state, the second station can accept packets from any stations. During the interval where the second station remains locked by the first station, when a third station transmits a packet to the second station, the transmitted packet is rejected by the second station. The third station is informed by the rejection of the transmitted packet that the second station is held by another station. In some cases, the third station abandons the communication with the second station. In other cases, the third station retransmits the packet to the second station at a moment a predetermined time after the first transmission of the packet.
In these prior art communication networks, while a first station remains locking a second station, a third station can not communicate with the second station. In addition, since the third station is not previously informed of the expected time of the end of the communication between the first and second stations, the third station tends to wastefully transmit a packet many times.